A reciprocating piston engine, in particular one for driving a motor vehicle, comprises cylinders with inlet and outlet valves, which control a gas exchange in the cylinders. A valve lift of an inlet or outlet valve can be modified by allowing actuation of the valve by means of different cams on a camshaft. For this purpose, there can be provided, for example, a bucket tappet which has several sections, each associated with one cam on the camshaft. By establishing or releasing an axial nonpositive engagement between the individual sections, one or more cams can be used to actuate the valve. It is thereby possible to modify valve timings, allowing the running behavior of the reciprocating piston engine to be adapted to an operating state or a load requirement, for example.
In one embodiment, an adjustable bucket tappet of this kind is controlled hydraulically. For example, first timings for the valve can be activated when a hydraulic pressure in a line assumes a first, low value and second timings can be activated when the pressure assumes a second, higher value. An electromagnetic hydraulic valve can be used to control the hydraulic pressure, for example. In other embodiments, the variable valve gear can also be directly adjustable by electric means.
Variation of the valve timings must be carried out in accordance with other operating parameters of the reciprocating piston engine. Otherwise, there is the risk that the reciprocating piston engine will be damaged by the modified timings, e.g. if there is a risk of collision between the piston and the open valve in the case of extended valve actuating times and high speeds of the reciprocating piston engine.
DE 197 36 064 A1 shows the use of a signal of a lambda probe in the exhaust duct of the reciprocating piston engine to monitor variable valve timing on the inlet side. A solution of this kind is not suitable for reciprocating piston engines which have a plurality of cylinders, the valves of which have different timings. This can be necessary, for example, when using two or more banks of cylinders, the exhaust gases from which are passed into a common exhaust section.
DE 20 2009 006 940 U1 proposes to monitor the magnetic field of an electrohydraulic changeover valve by means of a sensor in order to determine an axial position of an armature of the valve. However, the use of a separate sensor is often problematic for reasons of cost and reliability.
DE 10 2004 030 779 A1 relates to the monitoring of an electrohydraulic valve having a coil and an armature. When the coil is activated, it moves the armature, which is then held in its position by means of a permanent magnet. After the adjustment of the valve lift, the armature is pushed back mechanically into the coil, thereby inducing in the coil a voltage which is monitored. If the expected voltage fails to appear, a defect in the valve is identified. However, such a solution presupposes that a sufficiently long time has passed between the end of energization of the coil and the mechanical return of the armature to dissipate any magnetization of the coil. Otherwise, the voltage observed across the coil cannot be associated with certainty with a movement of the armature. Moreover, this method is suitable only for an electromagnetic changeover valve having a permanent magnet.